High-power, phase-locked, laser arrays

ABSTRACT

High-power, phased-locked, laser arrays as disclosed herein utilize a system of optical elements that may be external to the laser oscillator array. Such an external optical system may achieve mutually coherent operation of all the emitters in a laser array, and coherent combination of the output of all the lasers in the array into a single beam. Such an “external gain harness” system may include: an optical lens/mirror system that mixes the output of all the emitters in the array; a holographic optical element that combines the output of all the lasers in the array, and an output coupler that selects a single path for the combined output and also selects a common operating frequency for all the coupled gain regions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/346,667, filed Feb. 3, 2006, which claims benefit under 35 U.S.C.§119(e) of provisional U.S. patent application No. 60/649,489, filedFeb. 3, 2005, the contents of each of which are incorporated herein byreference.

BACKGROUND

Lasers are the brightest and, in many instances, some of the mostefficient sources of light available today. Nevertheless, there is aconstant need to increase their brightness and total power even further.There are, however, real barriers to increasing the brightness of asingle laser source of any design that currently present some of themost difficult technological challenges. One of the best examples issemiconductor lasers. Semiconductor lasers provide the highestelectrical-to-optical power-conversion efficiency, and achieve very highoptical gain, but typically do not provide sufficiently high contiguousgain volume. As a result, optical power in excess of 10 W (i.e.,continuous wave) can currently be extracted only from arrays ofsemiconductor laser diodes. Such ensembles of individual laser emitters,however, cannot readily provide a diffraction-limited or nearlydiffraction-limited beam, even though they can deliver very impressivetotal output power (e.g., ˜500 W/cm2 of array face area currentlyachieved). This results in poor brightness of otherwise highly efficientand powerful light sources.

The poor mode quality of laser diode arrays stems from the fact that thegain is extracted from physically separate pimp regions, essentiallymaking them extended area light sources. However, if there was atechnique to harness the gain of such separate gain regions and makethem behave as parts of a contiguous gain medium with a constant phasedifference between all the regions, then the light emitted by all suchregions would be coherent and, as a result, could be efficientlycombined into a single beam of much higher optical quality than that ofthe standard laser diode array. This problem is common to all lasertypes and, if solved, would allow for harnessing together the power ofmultiple laser sources with progressively increasing brightness, withouthaving to deal with the problems of scaling up the power of eachindividual source.

Other methods of beam combination for increased brightness includepolarization combining and spectral beam combining. Polarization beamcombining, however, can only increase brightness by a factor of two.Spectral beam combining, on the other hand, does not increase spectralbrightness.

SUMMARY

The approach disclosed herein utilizes a system of optical elements thatmay be external to the laser oscillator array. Such an external opticalsystem may achieve: a) mutually coherent operation of all the emittersin a laser array; and b) coherent combination of the output of all thelasers in the array into a single beam.

An example embodiment of such an “external gain harness” system mayinclude: 1) an optical lens/mirror system that mixes the output of allthe emitters in the array in one region in space; 2) a beam combiningelement (BCE), positioned in the region of space that combines theoutput of all the lasers in the array; and 3) an output coupler,positioned after the BCE, that selects a single path for the combinedoutput and also selects the common operating frequency for all thecoupled gain regions.

The optical mixing system may be a single lens. It should be understood,however, that the optical mixing system may include any combination ofone or more lenses, mirrors, and/or prisms. A BCE may be reflective ortransmissive, and may be manufactured using three-dimensional,holographic Bragg grating technology, such as VBG™, which is developed,manufactured, and distributed by PD-LD, Inc. It should be understoodthat a BCE may be manufactured using other techniques. For example, theBCE may be a holographic optical element, a diffractive optical element,such as a Dammann grating or a spot array generator, or any otheroptical element with suitable functionality. The output coupler may beconstructed from a reflective or transmissive Bragg grating element thatprovides wavelength-selective feedback for a single optical path of theBCE that forces coherent operation of all the emitters with a specificphase difference that will achieve constructive interference in thatparticular optical path. Other possible options include aphase-conjugate mirror used in combination with one or more opticaletalons or other wavelength-selective elements.

A principle of operation of such a “gain harness laser (GHL)” is inmixing the output of all the individual emitters in the array, filteringthe mixed output based on the relative phase of the light emitted byeach of the separate gain regions, then further filtering the lightbased on wavelength in order to limit the number of longitudinal modesoscillating in the composite resonator. The mixed and filtered output isthen fed back into the separate gain regions, each of the regions thusreceiving seed light from all the others with appropriate wavelength andphase. When the returned light is filtered in the way described, thefeedback from all the gain sections adds constructively at the frontemitting-aperture of each of the gain regions and, therefore, creates arelatively strong feedback capable of locking the laser array intocoherent operation.

The approach described herein may achieve long range coupling betweenthe members of a laser oscillator array, limiting the number of coupledmodes by using wavelength-selective feedback from a Bragg gratingelement, and also limiting the coupling to a particular phase state ofthe emitter array by use of a BCE and an optical path selector.

The GHL concept is, in principle, rather similar to the injectionseeding of a high-power slave laser with a lower-power master laser.However, unlike in the case of master/slave configuration, there is noneed for active phase control of the seed light—the phase adjustmenthappens automatically due to the effect of the gain harness.Furthermore, in case of GHL, there is no need to cascade the slavelasers/amplifiers into multiple stages—the seed power scales directlywith the number of lasers in the array. Nonetheless, the GHL itself canbe used as a powerful seed source for a large super-array ofsemiconductor or other type lasers in order to achieve higher-levelcoherence and overall power.

The GHL approach described herein may provide for self-adjustingcoherence (e.g., there may be no need for active phase control). Theremay be no need for an external seed source. There may be no need formodifying the high-power laser resonators, which allows the use oflasers with highest possible native efficiency. The approach providesfor simplicity and robustness (e.g., minimum, number of opticalcomponents), and for scalability—via using the GHL as a seed source andvia spectral combining. It should be understood that spectral beamcombining, which does not increase spectral brightness, may be employedas a complementary technique to the coherent beam combining describedherein.

To understand the functionality of the BCE, consider the propagation ofthe laser light in reverse. For example, if one considers adiffraction-limited beam entering the gain harness block of the GHLcounter-propagating with its output, that beam will pass through theBragg grating output coupler, with a certain percentage of it beingreflected back, and reach the BCE. The BCE will produce an imagematching the laser array that will be projected onto the outputapertures of the lasers by the Fourier-transform lens. Note that, for aproperly constructed BCE, each laser emitter may receive the sameproportion of light injected into the cavity. If the light returned fromthe laser cavities has the same phase difference between the differentpaths as the incident light, all the beams of light returning to the BCEwill be recombined into a single diffraction-limited beam nowcounter-propagating with the imaginary injected beam of light, thusclosing the cavity path.

The selection of the proper phase condition for the reconstruction ofthe output beam is likely to occur spontaneously in the GHL cavity dueto the dense mode spacing of the external gain harness relative to thespectral width of the reflectivity envelope of the output coupler.Although the use of a Bragg grating as an output coupler represents themost convenient and simple method of completing the GHL cavity, aphase-conjugate mirror (in conjunction with an output path selector,which may be a simple aperture, and a band pass filter, which may be anoptical etalon, thin film filter or other) may also yield good stabilityof the cavity.

The performance of the device may be related to the performance of theBCE. For example, the diffraction efficiency of the BCE and the amountof light leaking into higher orders may determine the total loss of theGHL cavity. The use of holographic Bragg grating elements forfabrication of a BCE may be desirable, therefore, as they afford thepossibility of fabricating a true matching filter for a particular laserarray, as opposed to a phase-only filter constructed using, for example,the techniques of binary digital holography employed for fabrication ofsurface-etched pattern generators. It should be understood, however,that such a BCE may be fabricated via techniques other than Bragggrating technology.

The technique described herein applies to many different kinds of laseroscillators and/or gain media: semiconductor laser diodes (bothoptically and electrically pumped, either single transverse mode orbroad-area multi-mode), solid-state lasers, fiber lasers, gas and ionlasers, atomic vapor lasers, etc. It is not restricted to strictly TEM₀₀operation of the oscillators.

There are at least two possible regimes for the operation of laserarrays with external gain harness—i.e., weak and strong couplingregimes. In the case of weak coupling, the laser array is operatingabove threshold near its nominal output power and pump level. Thefeedback from the external gain harness acts as a weak perturbation tothe array, just sufficiently strong enough to cause the spontaneoustransition to the coherent operation. In the case of strong coupling,however, the laser array would not nominally reach the lasing thresholdbut, rather, would be operating as an amplifier or an active mirror witha faceted surface. In that case, the external gain harness may completethe cavity and reduce the intracavity losses to the level sufficient forstable lasing.

The GHL concept may be, in many respects, similar to self-mode-lockingof ultra-fast lasers via the formation of a Kerr lens inside the gainmedium. The concept may be referred to as Kerr Lens Mode locking or KLM.Similar to GHL, KLM relies on the fact that the cavity loss becomessignificantly reduced for a particular phase difference between thelongitudinal modes of the laser cavity, namely that which leads to theformation of a continuous train of short pulses. In the case of GHL theparticular phase difference between the different gain paths leads tothe constructive interference along the path selected by the outputcoupler/path selector.

The GHL approach offers a clear and simple path to further power scalingdue to its inherent compactness and narrow-band operation. There are twopossible routes for GHL power scaling: 1) using GHL as a seed for alarger array of coherent lasers; and 2) using spectral combining of theoutput of individual GHLs. The first approach, although feasible, mayrequire active phase control on the separate injection paths and,therefore, may be rather complex. The second approach, on the otherhand, is a very natural choice because the output of GHL tends to bevery narrow spectrally. For this reason, a large number of GHLs can becombined spectrally within a spectral region of several nanometers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B depict a cavity with multiple gain paths phase-locked byuse of a beam combining element (BCE), a path selector, and differenttypes of output couplers.

FIG. 2 illustrates the role of a wavelength-selective output coupler.

FIGS. 3A and 3B depict laser cavities with phase locking ofmultiple-gain paths and different types of wavelength-selective, tunableoutput couplers.

FIGS. 4A and 413 depict a gain harness laser with long-range couplingbetween the laser array elements.

FIG. 5 provides a schematic of a test setup for inducing cross-elementcoherence of a laser diode bar.

FIGS. 6A and 6B depict the results of a test on inducing cross-elementcoherence of a laser diode bar.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

FIGS. 1A and 1B depict example embodiments of systems and methods forachieving phase synchronization of multiple gain paths of a complexlaser cavity. A composite resonator 100, as shown, may include aninternal part 102, which may include those components that a typicallaser cavity would have, and an external part 104, which may includeadditional components that enable phase synchronization of multiple gainpaths in the laser cavity. The internal part 102 may include an array120 of gain section resonators 121-125. Each gain section 121-125 mayemit light, such as laser light, for example, along a respective opticalpath 131-135.

In order to achieve the best phase-locking (i.e., coherence) between theseveral gain sections 121-125, it may be desirable that coupling,preferably equally-strong coupling, is achieved between each emitter121-125 in the array 120 and every other emitter 121-125 in the array120. In order to construct a complete laser cavity with the independentgain sections 121-125 of the array 120, a plurality 110 of highlyreflective mirrors 111-115 may be disposed proximate a first end of thegain sections 121-125. The opposite ends of the gain section 121-125 mayhave partially reflective mirrors (not shown) or no mirrors at all (asshown).

In order to achieve coupling between the individual gain sections121-125, it may be desirable to mix the optical paths of the light beams131-135 emitted by the gain sections 121-125. That is, the optical pathsmay be caused to overlap one another in space. As shown in FIGS. 1A and1B, this may be accomplished by a Fourier transform (FT) lens 140disposed to receive the light beams 131-135 emitted by the individualgain sections 121-125. The lens 140 performs an optical Fouriertransform on the received beams 131-135, forming transformed beams141-145. Each of the transformed beams 141-145 corresponds to arespective one of the emitted beams 131-135. The FT lens 140 focuses thetransformed beams 141-145 onto its back focal plane. The transformedbeams 141-145 are thereby mixed, in a location 147 in the back focalplane of the lens 140.

A beam combining element (BCE) 150 may be positioned in the back focalplane of the FT lens 140, and in the front focal plane of a second FTlens 160. The BCE 150 may be designed to split a single beam of light(e.g., beam 143) into a plurality of beams 151-155 in a controlledmanner. The BCE may split each incident beam into the same number ofbeams as there are gain sections in the array 121-125. At least one ofthe beams 151-155 output from the BCE should be a composite of all thebeams 141-145 and, therefore, representative of a composite of all thebeams 131-135. The beams 151-155 are split out of the BCE in such amanner as to form a pattern 157 that corresponds to a spatialarrangement of the individual gain sections 121-125 of the system. Thatis, the pattern 157 represents how the emitters 121-125 are arrangedrelative to one another (e.g., the beams output from the BCE form anarray that matches the array of beams output from the emitters).

The BCE may be designed to be made by using the techniques ofthree-dimensional Bragg gratings (described elsewhere), using surfacediffractive optical elements, or any other suitable technique. The BCEmay be made of an optical material with high transparency, highdurability, and high optical damage threshold.

The second FT lens 160 performs an optical. Fourier transform on thebeams 151-155 received from the BCE 150, forming re-transformed beams161-165. At least one of the re-transformed beams 161-165 is a compositeof all the beams 131-135. Accordingly, after the optical Fouriertransform is performed by the second FT lens 160, a pattern 167 willappear that includes an image 170 of the array 120 of individual gainsections 121-125. The images 171-175 of all the individual gain sections121-125 will overlap at least on one of the images of the exit aperturesof the individual gain sections.

A path selector 180 may be positioned after the second FT lens 160. Thepath selector 180 may define an aperture 182 that allows light from oneof the optical paths 161-165 (e.g., optical path 164, as shown) throughthe path selector 180. The path selector may be made of anon-transparent material that is robust enough to withstand the exposureto light at the operating power of the system. Thus, the path selector180 may restrict the feedback into the gain regions to come only from apath containing overlapping beams from all the gain sections in thearray. Together with the BCE 150, the path selector 180 may force aparticular phase state for the ensemble of phase-locked emitters thatwould produce constructive interference from all the emitters in theoutput of the system.

An output coupler 190 may be positioned behind the path selector 180.The output coupler 180 may reflect back some or all of the lightpropagating along the optical path 164 selected by the path selector180, thus completing the external portion 104 of the composite cavity100. As shown in FIG. 1A, the output coupler 190 may include athree-dimensional, Bragg grating element 192, which may be formed of athree-dimensional, transparent material (e.g. glass, crystal, polymer)having a Bragg grating recorded therein. Such a Bragg grating elementmay have a spectrally-narrow reflectivity band that tends to restrictthe number of longitudinal modes oscillating in the composite cavity 100and, as a result, tends to achieve stable phase locking between theindividual gain sections 121-125 of the array 120.

The BCE 150 re-maps the light returned by the output coupler 190 (alongthe optical path 164 selected by the path selector 180) back onto thearray 120 of gain sections 121-125. As an array 120 of individual gainsections 121-125 will have a fill factor of less than one (and verylikely much less than one), it may be desirable that the light returned,by the external part 104 of the composite cavity 100 does not fall inbetween the front emitting apertures of the individual gain sections121-125. This may be accomplished by a properly-designed BCE 150 thatinsures minimal possible loss inside the composite cavity.

The mixed and filtered output (reflected from the Bragg grating elementalong optical path 164) is thus fed back into each of the gain regions121-125. Each of the gain regions 121-125 thus receives “seed” lightfrom all the others (because the composite beam is fed back to each)with appropriate wavelength, selected by the output coupler, and phase,selected by the BCE and the path selector. When the returned light isfiltered as described above, the feedback from all the gain sections121-125 adds constructively at the front emitting apertures of theseveral gain regions 121-125 and, therefore, creates a relatively strongfeedback capable of locking the laser array 120 into coherent operation.Thus, the Bragg grating element provides feedback for a single opticalpath of the BCE that forces coherent operation of all the emitters witha specific phase difference that will achieve constructive interferencein that particular optical path.

As shown in FIG. 1B, the output coupler 190 may include a bandpassfilter 194 that allows a selected wavelength or band of light through toa mirror 196, which may be a phase-conjugate mirror or an ordinarymirror, for example. The mirror 196 reverses the phase of the lightincident upon it. Such an output coupler may be used in conjunction withother wavelength-selective elements or band-pass filters such as, forexample, optical etalons, Bragg gratings, diffraction gratings, etc.

Note that, although FIGS. 1A and 1B depict five separate gain sections121-125 arranged in a one-dimensional array 120, it should be understoodthat the techniques described herein apply, without restriction, tolarger-sized one-dimensional and two-dimensional arrays of emitters.

FIG. 2 illustrates the role of a wavelength-selective output coupler.The individual gain sections 121-125 of the emitter array 120 may haveinternal resonators formed by the highly reflective mirrors 111-115 nearthe respective back apertures of the gain sections 121-125 and partiallyreflective mirrors (not shown) near the respective front apertures ofthe gain sections 121-125. In general, each of the internal resonatorsmay have a slightly different mode comb 221-225. When awavelength-selective output coupler is used, it may restrict the numberof internal resonator modes to a single mode or a few close modes, asshown by the dashed ellipse in FIG. 2, thus increasing the coherencebetween the light 131-135 emitted by the individual gain sections121-125. Note that the lower the reflectivity of the partial reflectorin the front part of the gain section (not shown), the wider will be thebroadening of the modes within the mode comb of the internal resonators.This may facilitate better phase locking of the individual gain sections121-125.

FIG. 3A depicts a tunable, wavelength-selective output coupler 190 usinga transmissive Bragg grating element 192. If wavelength tuning isdesired in an array 120 of coherent emitters 121-125, it can beaccomplished via angular adjustment of the transmissive Bragg gratingelement 192. Adjusting the angle of incidence α of light onto atransmissive Bragg grating 194 changes the Bragg matching condition and,therefore, changes the wavelength for which maximum diffractionefficiency is achieved. Thus, the Bragg grating element 192 can providewavelength-selective feedback for a single optical path 164 of the BCE150 that forces coherent operation of all the emitters 121-125 with aspecific phase difference that will achieve constructive interference inthat particular optical path 164.

As shown in FIG. 3B, a diffraction grating 196 can be used in well-knownLittrow (as shown in FIG. 3B), Littman-Metcalf, or other configurations,in order to provide wavelength-selective feedback back into theemitters.

As shown in FIGS. 3A and 3B, the light 131-135 emitted by the individualgain sections 121-125 is mixed in the back focal plane of the first FTlens 140, where a BCE 150 is positioned. The BCE 150 mixes the opticalpaths 141-145 of all of the gain sections 121-125. A second FT lens 160may be used in order to produce mixed images 171-175 of the individualgain sections 121-125 in its back focal plane. A path selector 180 maybe positioned in the back focal plane of the second FT lens 160. Thepath selector 180 selects a single common optical path 164 for alllight-emitting sections. A transmissive Bragg grating element 192diffracts the mixed light at a certain diffraction angle β according toparticular design conditions. A partially or fully reflective outputcoupler 194 is positioned in the path of the diffracted beam 193. Theoutput coupler 194 reflects the light incident upon it back on its path164, thus completing the external part 104 of the composite resonator100. Wavelength tuning is achieved by adjusting the angle α between theBragg grating element 192 and the beam incident upon it.

Similarly, as shown in FIG. 3B, a diffraction grating 196 can berotated, which adjusts the wavelength of the light returned onto theemitters. Thus, the diffraction grating 196 can be used as awavelength-selective element that can achieve wavelength tuning viaadjustment of the incident angle δ. The diffraction grating 196 may bereflective, as shown in FIG. 3B, or transmissive. It may be a surfacediffraction grating or a volume holographic grating. The diffractiongrating 196 can be manufactured by any of a number of techniques, suchas, for example, surface ruling, holographic techniques, etching, etc.The wavelength-selective feedback may be produced in the 1^(st) orhigher diffraction order of the diffraction grating, and the output ofthe system may be produced in the 0^(th) order of the diffractiongrating.

FIGS. 4A and 4B depict a gain harness laser (GHL) 400 with long-rangecoupling between the laser array elements 421-425. The GHL 400 mayinclude an array 420 of individual gain sections/emitters 421-425, whichmay be an array 420 of laser diodes. It may also include a Fouriertransform (FT) lens 430, a BCE 440, a path selector 450, and awavelength-selecting element 460, such as a three-dimensional Bragggrating element (as shown in FIG. 4A) or a diffraction grating (as shownin FIG. 4B) as an output coupler. The FT lens 430 collimates andoverlaps the outputs of the individual emitters 121-125 in a certainlocation in space. The BCE 440 may be positioned in that location andreceives the focused optical paths 421-425. The BCE 440 mixes thefocused optical paths 431-435 of all of the gain sections 421-425 into acommon path. As shown in FIGS. 4A and 4B, a path selector 450 ispositioned behind the BCE 440, without the use of a second FT lens (asdescribed above). The light allowed through the path selector 450 isreflected back by a three-dimensional Bragg grating (as shown in FIG.4A) or a diffraction grating (as shown in FIG. 4B) that serves as awavelength-selective output coupler 460 due to its narrow reflectivityspectrum. The diffraction grating may be reflective, as shown in FIG.4B, or transmissive. It may be a surface diffraction grating or a volumeholographic grating. The diffraction grating can be manufactured by anyof a number of techniques, such as, for example, surface ruling,holographic techniques, etching, etc. it may be used in any well-knownarrangements, such as Littrow, Littman-Metcalf, or any other suitableconfiguration.

FIG. 5 is a schematic diagram of a test setup 500 for inducingcross-element coherence of a laser diode bar using a Talbot cavity. Asshown, the test setup 500 may include an array 520 of individual gainsections/emitters 521-525, a collimating lens 530 or lens array, and athree-dimensional Bragg grating element 540 that reflects some of thelight back into the individual emitters 521-525.

An experiment was conducted using the test setup shown in FIG. 5. TheBragg grating element 540 was positioned at a particular distance fromthe face of the laser diode bar 520 so that, upon reflection from theBragg grating element 540, the diffraction pattern on the face of thelaser diode bar 520 would repeat or nearly repeat the arrangement of theemitters 521-525 and, therefore, maximize the coupling in the compositecavity. A Fourier-transform lens 550 was used to produce the far fieldpattern of the array, which was observed on a screen 560. If coherencebetween the emitters 521-525 in the laser diode array 520 is achieved,the far field pattern will show clear signs of coherent interference(i.e., fringes).

FIGS. 6A and 6B provide far field patterns observed during theexperiment. FIG. 6A provides a far field pattern for a situation whereinthe Bragg grating element 540 is aligned to reflect the incident lightback on its path. FIG. 6B provides a far field pattern for a situationwherein the Bragg grating element 540 is iris-aligned and, therefore,does not reflect the incident light back on its path. The results showan apparently stable interference pattern in the far field of the laserdiode array 520, which is only possible if there exists a constant andstable phase difference between the modes of the several emitters521-525 in the array 520. If the phase difference between the individualemitters 521-525 in the array 520 is not constant, then the interferencepattern loses it stability and visibility (i.e., no periodic darkfringes are observed).

It should be understood that the systems and methods described andclaimed herein may be applied to, among other things: apparatus andmethods using reflective and transmissive holographic Bragg gratingelements as a feedback element to achieve coherence; systems usingcoherently combined laser arrays that use Bragg grating elements;systems that perform spectral beam combining of the coherently combinedlaser arrays using Bragg grating elements; coherently combined systemsusing Bragg grating elements as wavelength selector that are used forsecond harmonic generators or optical parameteric oscillators orparameteric amplification; coherent combining of laser diodes,solid-state lasers, fiber lasers, gas lasers, ion lasers, alkali vaporlasers, and the like; coherent combining of lasers with TEM₀₀ output;coherent combining of lasers with multi-mode output; and using phaseconjugate mirrors for coherent beam combining.

The advantages of laser systems constructed according to the approachesdescribed herein can be exploited in any application that benefits fromlaser sources with increased brightness and power. Such applicationsinclude, but are not limited to, laser pump sources, direct materialprocessing, military applications (e.g., directed energy weapons, targetdesignators, laser range finders, etc.), laser radars, opticalcommunications, spectroscopy (including differential absorptionspectroscopy, Raman spectroscopy, different other nonlinear spectroscopytechniques), medical applications (e.g., therapeutic, surgical,diagnostic, etc.), remote sensing, security applications, etc.

1. A method for producing mutually-coherent operation of a plurality of light emitters, each of the emitters outputting a respective power and brightness, the method comprising: mixing respective beams received from each of the plurality of emitters; combining the mixed beams in a beam-combining element to form a composite beam containing a respective contribution from each of the emitters; and feeding back through the beam-combining element at least a portion of the light containing the respective contribution from each of the emitters, the portion having a selected wavelength, wherein the portion is fed back into the emitters in such a way that each of the emitters receives narrowband light from every other emitter, and wherein mutually coherent operation of the plurality of emitters is achieved such that each of the emitters lases at the selected wavelength with a phase difference that results in a coherent output beam that effectively combines the powers from each of the emitters such that the output beam has a brightness that is greater than the brightnesses of the individual emitters.
 2. The method of claim 1, further comprising: mixing the beams received from the emitters by overlapping the received beams in a focal plane of a lens.
 3. The method of claim 2, wherein the lens performs an optical Fourier transform on the beams received from the emitters.
 4. The method of claim 2, wherein the beam-combining element is positioned in the focal plane of the lens.
 5. The method of claim 4, wherein the beam-combining element is a diffractive optical element.
 6. The method of claim 4, wherein the beam-combining element is a digital optical element.
 7. The method of claim 4, wherein the beam-combining element is a holographic optical element.
 8. The method of claim 2, wherein the beam-combing element is positioned in a focal plane of a second lens.
 9. The method of claim 1, further comprising: allowing the composite beam to pass through a path selector.
 10. The method of claim 9, wherein an output coupler reflects at least a portion of the composite beam back through the path selector.
 11. The method of claim 10, wherein the output coupler comprises a three-dimensional optical element having a Bragg grating recorded holographically therein.
 12. The method of claim 11, wherein the Bragg grating is a reflective Bragg grating.
 13. The method of claim 11, wherein the Bragg grating is a transmissive Bragg grating.
 14. The method of claim 10, wherein the output coupler comprises a phase-conjugate mirror.
 15. The method of claim 10, wherein the output coupler comprises a diffraction grating.
 16. The method of claim 10, wherein the output coupler is a tunable output coupler that provides for wavelength tuning of the reflected portion of the composite beam.
 17. The method of claim 16, wherein the tunable output coupler provides for the wavelength tuning via angular adjustment of the output coupler.
 18. The method of claim 1, wherein the respective contributions from each of the emitters are received by a three-dimensional optical element.
 19. The method of claim 18, wherein the composite beam is created via an optical method that includes the three-dimensional optical element.
 20. The method of claim 19, wherein the composite beam is created via an optical method that includes the three-dimensional optical element receiving the beams from the emitters and forming the composite beam containing the respective contributions from each of the emitters. 